Is it better to set default to Null for a certain field in the database table, or '', or Empty String? On the mobile phone, I just want to return empty

When a certain field in the database is set to Null, the mobile terminal will return Null by default. If I want the mobile terminal to return null, I have to make a judgment.
1. If I want the mobile phone to return empty without making any judgment, how should I set the table field default?
2. Is it better to set the table field default to Null, "", or Empty String? Are they different?